Mouse cells transformed in vitro with SV40 virus are, in general, rapidly rejected in vivo. Evidence from other laboratories suggests that rejection is mediated by a T-lymphocyte response to the SV40-specific transplantation antigen. We and others find both T\cytotoxic cells (CTL) and delayed hypersensitivity (DTH) responses to SV40 cells, but the relative role of these mechanisms in tumor rejection is unknown. We have recently isolated variants of an SV40-transformed C3H cell line that differ in their susceptibility to lysis in vitro with syngeneic anti-SV40 CTL or LPS-activated macrophages. The lines present a unique opportunity to investigate the cellular mechanisms involved in rejection of SV40 tumors. Based on these studies, the working hypothesis is proposed that while either CTL or DTH mechanisms can produce secondary rejection of SV40 tumors in SV40-immune mice, both mechanisms acting in concert are necessary for primary rejection of tumors in nonimmune recipients. To test this hypothesis, cloned SV40 lines of differing immunosensitivities, as determined by in vitro assay, will be tested for growth in naive and immune recipients. These experiments will employ strains of mice previously found to be CTL responders or nonresponders to SV40. We have developed an in vitro assay for Lyt 1+ recognition of SV40 cells which relies on the ability of activated Lyt 1+ cells to elaborate macrophage activating factor (MAF). Furthermore, we have shown that the tumor-specific production of MAF results in tumoricidal macrophages both in vivo and in vitro. We are currently performing adoptive transfer experiments to elucidate the relative roles of CTL and activated macrophages in tumor growth inhibition in vivo. The overall goal of these studies is to begin to dissect tumor-eliminating T-cell responses in vivo to a single tumor-associated antigen (SV40 T\antigen) as a function of the genetically determined ability to respond to that antigen.